1. Field of the Invention:
This invention relates to an alcohol engine with combustion cavity which allows main combustion chambers to communicate with combustion cavities via communication ports and is equipped with fuel injection nozzles in the combustion cavities.
2. Description of the Prior Art:
Environmental pollution by exhaust gases emitted from engines has become a serious problem in recent years and alcohol engines have attracted increasing attention. In the alcohol engine, the contents of carbonic acid gas and carbides in the exhaust gas are extremely small in comparison with other fuels such as gasoline and gas oil. A Diesel engine using alcohol fuel however involves a problem of poor ignitability. In other words, alcohol has larger latent heat for vaporization than gasoline. For example, whereas gasoline requires latent heat of vaporization of 0.7% of the fuel, alcohol requires latent heat of vaporization of 5% of the fuel; hence, the alcohol fuel is difficult to vaporize. Moreover, the alcohol fuel injected from fuel injection nozzles into compressed air inside the combustion chambers lowers the temperatures of compressed air and wall surfaces of the combustion chambers due to vaporization and thus reduces ignitability.
A laminar intake engine system utilizing a collision injection flow of a fuel, or in other words, a methano engine having combustion chambers of a fuel collision diffusion system, which forms a collision portion by sticking out the piston head center portion of a piston and forms a recessed cavity around this collision portion is known (e.g. "Internal Combustion Engine", Vol. 27, No. 345, July, 1988, pp. 35 to 42). In the internal combustion engine having these combustion chambers, the fuel injected from a single-hole nozzle of the fuel injection nozzles is caused to impinge against the projection portion of the piston head and is diffused in a disc-like form. Then, the fuel is mixed with air and forms a fuel-air mixture while being entrapped at the lower part of the cavity by the squish flow generated by the rise of the piston.
In the fuel injection nozzles, the nozzles having the injection ports which are formed in two stages and multiple injection ports are disclosed, for example, in Japanese Patent Laid-Open No. 88413/1978 and Japanese Utility Model Laid-Open No. 202768/1988.
However, the methanol engine using the pistons equipped with the fuel collision diffusion type combustion chambers described above involves the problem that, when a very small quantity of fuel is injected from the fuel injection nozzles at the time of a partial load, dispersibility of the fuel is poor and carbon is deposited on the collision portion.
The calorific value of alcohol fuel is about 6000 Kcal/kg and this value is about a half of that of light oil. For this reason, the fuel injection quantity of the alcohol fuel must be greater than that of the light oil. When the fuel consisting primarily of the alcohol fuel is burnt in the engine in the diesel cycle, therefore, the combustion process, the combustion chambers, the injection system, and the like, are different from those of ordinary engines because the heat of vaporization is great, the calorific value is small, and so forth. In the alcohol engine using the alcohol as the fuel, the behavior of combustion at the time of the partial load is remarkably different from that at the time of high load.
When methanol fuel is used as the alcohol fuel, for example, the heat of compressed air is deprived due to vaporization of the fuel in the alcohol engine because the alcohol has a large heat of vaporization as described above and unburnt intermediate products are exhausted in many cases. Therefore, the following two main factors must be solved with the alcohol engine.
The first factor is that the combustion chamber must be of heat-insulating structure and the temperature rise of the combustion chamber wall high in order to supply the energy of vaporization to the atomized alcohol fuel at the time of a low temperature such as in the partial load operation of the engine. In the heat-insulating engine, however, the combustion chamber wall does not reach a high temperature so much at the time of partial load, so that ignitability and combustion are not good, misfiring and incomplete combustion takes place and, unburnt gas is likely to occur. Because the alcohol fuel needs large latent heat for vaporization, the inner wall surface of the combustion chamber does not reach a high temperature during the low speed low load operation of the engine. Therefore, the wall surface cannot absorb from the fuel the heat of vaporization and promote vaporization, and a vaporization mixture cannot be obtained easily. As a result, combustion becomes incomplete and the generation of unburnt gas is remarkable.
The second factor is that the wall member of the combustion chambers is cooled by the alcohol fuel at the time of high temperature such as in the full load operation of the engine and the temperature of the combustion chambers is not kept at very high temperature. In the heat-insulating engines, however, the wall of the combustion chambers attains a high temperature at the time of the high load. For this reason, though ignition and combustion of the fuel are effected satisfactorily, suction efficiency is likely to drop and abnormal ignition and combustion is likely to occur if the combustion chambers is at an excessively high temperature.